MANUAL TIME CALIBRATION IN JT44
BY HAL LUND ZS6WB
When using the WSJT JT44 mode and to a less extent the FSK441 mode, timing
is extremely critical. When using JT44 for terrestial work timing of the two
stations involved must be within about 1.5 seconds or the signal will not decode
The best solution is to use a program such as Dimension 4 to synchronize your PC to a time standard and remain on the internet while using JT44. If operating from a remote location where this is not an option here is a quick way of determining if your PC clock is close enough to work a station using the JT44 mode provided the other station is audible.
If the other station is transmitting First Period you should hear his transmission start at 03 seconds past the minute and end at 26 seconds past the minute. If he is transmitting Second Period his transmission will start at 33 seconds and end at 56 seconds past the minute. Use this as a reference to reset your PC clock and then use the Dsec adjustment in the JT44 program which allows you to increment or decrement time up to 10 seconds for fine adjustment.
PC clocks drift considerably and it is best to have an external digital clock that you can check the PC time against when you are not connected to the internet. I use an inexpensive Electronic Clock-Timer manufactured by Smiths for home and kitchen use and it does a good job, but it needs to be reset every two or three days.
I often hear stations calling on JT44 with up to 15-20 seconds timing error and there is no way that a timing error of this magnitude can be compensated for. The calling station has the responsibility to make sure that his clock is at least close enough for the station being called to be able to adjust with the Dsec control until the calling station can be decoded.
The method just described can be used for your initial QSOs and in emergencies when timing for PC synchronization is not available. JT44 was developed for weak signal detection and integration to use in EME and extended tropo work where signals are often not audible. Obviously if you can't hear the signal you can't manually set your time, so if you are planning to do any serious DX work, it is to your advantage to ensure that your timing is correct.
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